Ice banks are coolness storage devices wherein a phase-change material (PCM) is alternately frozen and thawed by brine circulated through tubes immersed in the PCM. In air conditioning systems and other cooling-cycle apparatus a substantial energy savings can be achieved by chilling the brine to freeze the PCM over time during off-peak load conditons and withdrawing the stored coolness for use during peak-load conditions. See for example U.S. Pat. Nos. 4,671,347 and 4,954,278.
Typically an ice bank is a covered cylindrical container in which an expansion space is provided above the brine tubes immersed in the PCM. A liquid PCM expands as it freezes and if it is water, as is most frequently the case, it will undergo an expansion of 9% since liquid water occupies only 91% of its volume when frozen. As the PCM freezes around the brine tubes its upper liquid level therefore rises and must occupy the expansion space.
If the frozen PCM around the tubes is sub-cooled well below its freezing temperatures for a prolonged period the liquid PCM in the expansion space above it will also freeze. This presents a problem later when coolness is being withdrawn from the ice bank, as in peak-load conditions. The relatively warm brine to be cooled by circulation through the tubes will eventually thaw all of the PCM around the tubes, whereas in the expansion space above where there are no brine tubes the frozen PCM will remain as ice long after the PCM around the brine tubes below is fully thawed. This leaves what is called an ice-cap in the expansion space which is quite difficult to eliminate. An ice-cap depletes the liquid PCM available for maintaining all the brine tubes immersed and thereby causes air to come between the solid PCM and the tubes, which greatly decreases heat transfer. In some cases electric warm air blowers have been untilized to thaw ice-caps but it may take as much as one or two days to accomplish full thawing.
One method of ice-cap prevention is to insure that the chiller is operated to sub-cool the brine in response to the temperature of the brine leaving the ice bank and returning to the chiller. The chiller is run at full load during the freezing cycle until the return brine temperature reduces to approximately 27.degree. F. or 28.degree. F. At that point the chiller is shut down and the brine supply temperature is then at about 22.degree. F. or 21.degree. F. By carefully avoiding excessive sub-cooling of the ice in this fashion an ice-cap will not form in the expansion space under most circumstances. The disadvantage, however, is that close monitoring of the brine return temperature is necessary. If the chiller is not turned off or fails to turn off at a return brine temperature of 28.degree. F. and instead continues to operate until the return brine temperature is about 23.degree. F. it is virtually certain that over a number of cycles an ice-cap will form.
It is the principal purpose of the present invention to provide fail-safe apparatus which can be certain to prevent formation of ice-caps notwithstanding excessive sub-cooling of the frozen PCM (water) even though the frozen PCM may be chilled to as low as 15.degree. F.